1. Field of the Invention
This invention relates to an electronic endoscope comprising a flexible conduit or scope and a video-signal processing unit to which the flexible scope is detachably connected at a proximal end.
2. Description of the Related Art
In such an electronic endoscope, the flexible scope includes an objective lens system provided at the distal end thereof, and a solid image sensor, such as a CCD (charge-coupled-device) image sensor, associated therewith. The flexible scope also includes an optical light guide extended therethrough, formed as a bundle of optical fibers, which is associated with a lighting lens system provided at the distal end of the flexible scope.
On the other hand, the video-signal processing unit includes a white-light source, such as a halogen lamp, a xenon lamp or the like. When the flexible scope is connected to the video-signal processing unit, the proximal end of the optical light guide is optically connected to the light source. Thus, an object to be photographed is illuminated by light radiating from the distal end of the optical light guide, and is focused as an optical image on a light-receiving surface of the CCD image sensor by the objective lens system.
The focused optical image is converted into a frame of analog image-pixel signals by the CCD image sensor. Then, the frame of analog image-pixel signals is read from the CCD image sensor, and is fed to the video-signal processing unit, in which the image-pixel signals are suitably processed, thereby producing a video signal including image-pixel signals and various synchronizing signals. Then, the video signal is fed from the video-signal processing unit to a TV monitor to reproduce the photographed object on the monitor on the basis of the video signal.
In general, the objective lens system, used in the electronic endoscope, exhibits a large depth of focus, because a close-range object image and/or a distant-range object image to be photographed must be focused on the light-receiving surface of the CCD image sensor by the objective lens system, before the photographed close-range object image and/or distant-range object image can be sharply reproduced on the monitor.
In this case, to maintain a constant overall luminance of a reproduced object image on the monitor, the radiation of the illuminating-light from the distal end of the optical light guide should be regulated in accordance with a distance between the photographed object image and the distal end of the optical light guide. For example, when only a medical image of a patient is to be reproduced as a close-up image by placing the distal end of the flexible scope close to the medical image, the radiation of the illuminating-light should be lowered to a minimum level in order to generate the medical image at a predetermined luminance on the monitor. Then, as the distal end of the flexible scope is moved away from the medical image, the radiation of the illuminating-light should be gradually increased from the minimum level to prevent the luminance of the reproduced medical image from being reduced.
Conventionally, for a regulation of radiation of the illuminating-light from the distal end of the optical light guide, an aperture-stop is associated with the white-light source, and is automatically controlled such that an overall luminance of the reproduced object image is always maintained at a constant level. In particular, a frame of luminance signals is extracted from the video signal at given regular time-intervals, and an average luminance level is calculated from the extracted luminance signals. Then, the radiation of the illuminating-light from the distal end of the optical light guide is regulated by controlling the aperture-stop such that the average luminance level coincides with a predetermined reference level.
In the conventional automatic control of the overall luminance of the reproduced object image, a localized halation frequently occurs on the reproduced image displayed on the monitor. Note, in this field, a halation is defined as a phenomenon in which a luminance level of a reproduced image on a monitor becomes abnormally high to be thereby whitened.
In particular, when an object image having a localized protrusion is photographed by the CCD image sensor such that the localized protrusion is close to the distal end of the flexible scope, and such that the remaining area of the object image is relatively far from the distal end of the flexible scope, a localized halation may occur at a localized area of a reproduced image on the monitor, corresponding to the localized protrusion of the photographed object image. This is because, although an aperture area of the aperture-stop has a tendency toward being widened due to the remaining area of the object image being relatively far from the distal end of the flexible scope, the localized protrusion is close to the distal end of the flexible scope.
Of course, the localized halation should be prevented, because a proper and detailed view of the reproduced image on the monitor is hindered due to the occurrence of the localized halation.
Therefore, an object of the present invention is to provide an electronic endoscope, which is constituted such that an occurrence of a localized halation on a reproduced object image displayed by a monitor can be effectively prevented.
In accordance with the present invention, there is provided an electronic endoscope which comprises a flexible scope having an image sensor provided at a distal end of the scope, and a video-signal processing unit to which a proximal end of the flexible scope is connected. Preferably, the connection of the distal end of the flexible scope to the video-processing unit is detachable. Image-pixel signals, successively read from the image sensor, are processed by the video-signal processing unit. A light source is provided in the video-signal processing unit such that light, emitted from the light source, is guided through the flexible scope and radiates from the distal end of the flexible scope. A light-emission regulator is associated with the light source to regulate the radiation of light from the distal end of the flexible scope. The electronic endoscope further comprises a histogram generator that successively generates a luminance-signal-histogram in accordance with the processed image-pixel signals, a first controller that controls the light-emission regulator in accordance with the luminance-signal-histogram, such that an image having a constant luminance level is reproduced in accordance with the processed image-pixel signals, a determiner that determines whether a localized halation occurs on the reproduced image on the basis of the luminance-signal-histogram, and a second controller that controls the light-emission regulator to forcibly decrease the radiation of light from the distal end of the flexible scope, thereby avoiding the localized halation occurrence.
In the histogram generator, the generation of the luminance-signal-histogram by the histogram generator may be based on either a frame of image-pixel signals or a field of image-pixel signals, extracted from the processed image-pixel signals. The first controller may include a calculator that calculates an average luminance level-value in accordance with luminance signals representing luminance levels of the luminance-signal-histogram. In this case, the light-emission regulator is controlled by the first controller such that the average luminance level-value coincides with a given reference-luminance-level-value. Preferably, the electronic endoscope may be provided with a manual adjuster that adjusts a magnitude of the reference-luminance-level-value.
Preferably, the electronic endoscope further comprises a monitor that monitors whether a difference between the average luminance level-value and the reference-luminance-level value is more than a predetermined relatively-large threshold value over a given time period after the localized halation occurrence, and a halation-avoidance-determiner that determines that the localized halation occurrence is avoided when it is confirmed by the monitor that the difference is more than the predetermined relatively-large threshold value over the given time period.
The determiner includes a frequency calculator that calculates a specific frequency of luminance signals, included in a specific range of the luminance-signal-histogram bounded by a maximum luminance level and a given specific luminance level thereof, and determines whether the localized halation occurs based on the specific frequency of luminance signals.
In accordance with an aspect of the present invention, the determiner includes a ratio calculator that calculates a ratio of the specific frequency of luminance signals to a total number of luminance signals included in the luminance-signal-histogram, and a comparator that compares the ratio with a given threshold value. In this case, it is determined by the determiner that the localized halation occurs when the ratio exceeds the threshold value.
The determiner may further include a numerical estimator that numerically estimates a magnitude of the ratio, and a halation-degree-determiner that determines a degree of the localized halation in accordance with the numerical estimation of the magnitude of the ratio. In this case, the forcible decrease in the radiation of light from the distal end of the flexible scope is performed by the second controller in accordance with the degree of the occurrence of the localized halation.
In accordance with another aspect of the present invention, the determiner includes a comparator that compares the specific frequency of luminance signals with a given threshold value. In this case, the determiner determines that the localized halation occurs when it is confirmed by the comparator that the specific frequency of luminance signals exceeds the threshold value.
In accordance with yet another aspect of the present invention, the determiner includes an indicator that indicates the localized halation occurrence. In this case, the forcible decrease in the radiation of light from the distal end of the flexible scope is performed by the second controller when the localized halation occurrence is indicated by the indicator.